The present invention relates to the conditions for connecting to the network electrical energy production sites and relates more particularly to improving the conditions of connection to the network of high power alternators.
The connection to the network of a generator of the mentioned type is ensured by means of a transformer.
The generator delivers to the network the active power and the reactive power.
The active power Pg supplied by the alternator is given by the relationship:
Pg=xe2x88x92Vgxc2x7Ixc2x7cos "psgr"gxe2x80x83xe2x80x83(1)
in which
Vg is the voltage at the terminals of the generator,
I is the current,
cos "psgr"g is the power factor of the generator.
The active power Pcp received by the network at the point of delivery is given by the relationship:
Pcp=+Vcpxc2x7Ixc2x7cos "psgr"cpxe2x80x83xe2x80x83(2)
in which
Vcp is the voltage at the terminals of the network, at the output of the transformer,
I is the current,
"psgr"cp is the power factor seen by the network at the point of delivery.
Pcp=xe2x88x92Pgxe2x80x83xe2x80x83(3)
It will be noted that given the above:
Vgxc2x7cos "psgr"g=Vcpxc2x7cos "psgr"cp
It will therefore be seen that the active power received by the network is equal to the active power supplied by the alternator, considering the losses of the transformer to be negligible.
The reactive power Qg of the generator is given by the relationship:
Qg=xe2x88x92Vgxc2x7Ixc2x7sin "psgr"gxe2x80x83xe2x80x83(4)
The reactive power of the network is given by the relationship:
Qcp=Vcpxc2x7Ixc2x7sin "psgr"cpxe2x80x83xe2x80x83(5)
Qgxe2x88x92XtI2=Qcpxe2x80x83xe2x80x83(6)
in which
Xt is the resistance of the transformer.
Thus, the reactive power received by the network is equal to that supplied by the alternator, decreased by the consumption of the transformer of reactive power XtI2.
However, the active power and the reactive power of the network supplied by the generator are constant values that the supplier of electrical energy has to take account of for the construction and the operation of the generator.
A schedule of charge established by the study commission EDF/GS21 defines the conditions of connection of private energy production units to the public transport network.
The principal rule concerning the electrical dimensioning of an installation for energy production is the following:
verification of the maximum short circuit current supplied by the installation at the point of delivery to the network, this verification having the result in most cases of giving rise to an increase of the reactance of the transformer to reduce the short circuit current of the installation,
verification of the installation to supply to the delivery point, a certain quantity of reactive power as a function of the conditions (value) of voltage at the delivery point.
This supply constraint is expressed in the document GS21 by a graph on which is shown on the abscissa the reactive power Q exchanged at the point of delivery, expressed in proportion to the maximum active power of the installation and officially called Pmax, and on the ordinate, the voltage at the point of delivery expressed as a value relative to a voltage reference, officially called Vref whose value is given by EDF.
The above verification and the resulting optimizations have naturally contradictory effects.
Observing the initial constraint of short circuit current tends very often to increase the reactance of the transformer connecting the alternator to the network.
The increase of this reactance leads to:
higher reactive power consumption in the transformer, which is undesirable when it is sought to export this reactive power to the network,
a greater and greater incapacity when the reactance increases, to:
export the active power when the voltage at the point of delivery is somewhat raised,
import reactive power when the voltage at the port of delivery is somewhat low.
The degradation of the performances connected to the reactance of the transformer is to be seen from an examination of two graphs.
The performances [q,u] at the terminals of the alternator which could be called gross,
The corresponding net performances [q,u] at the point of delivery.
The graph of gross performances [q,u] at the terminals of an alternator is a rectangle.
By way of example, there can be given:
a variable height of umin=0.9 uN to umax=1.1 Un
a width of Qmin=xe2x88x920.2 Pmax+0.6 Pmax
The graph at the beginning of the demonstration in the schedules of charges is based on an alternator whose diagram [q,u] at its terminals is a rectangle.
The graph of net performances [q,u] at the point of delivery becomes a trapezoid.
The initial rectangle containing all the points for which an operation of the generator is guaranteed, is naturally deformed when traversing the reactance of the transformer.
The high and low horizontal limits indicating the maximum or minimum constant voltage of the alternator are inclined and at the secondary of the transformer remain substantially straight.
They represent conditions of voltage and conditions of reactive power existing at the secondary of the transformer for a same condition of operation of the alternator.
In practice, the high and low limits are naturally obtained when the regulator of the alternator is blocked in the respective position of maximum voltage and minimum voltage and the voltage of the network at the point of delivery is varied to obtain the same reactive power absorbed or supplied by the alternator.
There will thus be seen an inability of the xe2x80x9calternator+transformerxe2x80x9d assembly to supply, or to consume, reactive power for the same voltage conditions.
Differences in voltage at the point of delivery are required which are much greater, to exchange the same quantities of reactive power.
The greater the reactance of the transformer, the greater the differences must be.
Increase of the reactance of the transformer is given on the graph by an increase of the slope of the trapezoid,
The invention seeks to overcome the mentioned drawbacks of the connection to the network of large machines and to permit improving the conditions of connection of such machines to the network.
It therefore has for its object a process for supplying electrical energy to the network with an AC generator associated with a turbine, by means of a transformer, characterized in that it consists in evaluating the generator precisely as a function of the on-site needs by lowering its nominal voltage as low as possible while continuing to satisfy the capacity of the generator to send active power delivered by the turbine and the reactive power corresponding to this active power required by a consumer or the manager of the network.
According to particular features of the invention:
lowering the voltage is carried out such that the generator can always transformer precisely the active power of the available turbine or dimension by conditions at the site and that the generator can simultaneously deliver precisely the associated reactive power,
the generator is suitable for a reduced voltage Ungf so as to reduce the short circuit current and to increase the dynamic of the range of regulated voltages at least equal to that of its previous range about its previous nominal voltage Ungi;
the generator being overdimensioned and initially designed for operation under a voltage Ungi, its operating voltage Ungf with a reduced short circuit current is connected to the initial operating voltage Ungi by the relationship:       Ungf    Ungi    =            Sngf      Sngi        =                                        Ps            2                    +                      Qs            2                              /                                    Png            2                    +                      Qng            2                              
xe2x80x83in which
Sg={square root over (Ps2+L +QS2+L )}
Ps and Qs being respectively the active power and the reactive power of the site,
and Png and Qng being respectively the nominal active power and the nominal reactive power of the generator;
the reduction of operating voltage of an existing generator is ensured by reduction of its excitation current and adjustment of the latter as a function of the new nominal function;
the re-evaluation of the generator is ensured by reducing its dimensions (in the case of a major re-evaluation).